The present invention relates to a process for the selective catalytic reduction of nitrogen oxides contained in a lean-mix exhaust gas from combustion engines by reducing the nitrogen oxides by means of ammonia on a catalyst.
Combustion engines with a lean exhaust gas concern diesel engines and lean-running petrol engines, i.e. so-called lean-mix engines. Compared to stoichiometrically operated conventional engines, diesel engines and lean-mix engines are characterized by an up to 20% lower fuel consumption. A substantial problem of these engines is the purification of their exhaust gases. Although the oxidizable harmful components of the exhaust gas (hydrocarbons HC, carbon monoxide CO and minor amounts of hydrogen H2) can on account of the high oxygen content in the exhaust gas of up to 15 vol.% easily be converted on a catalyst to carbon dioxide and water, the nitrogen oxides NOx however that are also formed in the combustion of the fuel cannot, on account of the preferably occurring oxidation reactions, be reduced in sufficient amount to nitrogen N2.
In order to solve this problem the process of selective catalytic reduction (SCR) already known in the case of stationary combustion units has been proposed. In this process a reducing agent is added to the lean exhaust gas, by means of which the nitrogen oxides can be selectively reduced on a catalyst suitable for this purpose. Ammonia, which reacts with a high selectivity with the nitrogen oxides to form nitrogen and water, is preferably used as reducing agent. The ratio of added ammonia to the nitrogen oxides that are present is about 1:1. Ammonia can be produced directly from urea with the aid of a hydrolysis catalyst or by decomposition of a corresponding salt (e.g. carbamate).
At the present time much effort is being expended on attempts to incorporate such systems in lorries and trucks. A disadvantage of this process is that a further operating material has to be employed. The high expenditure associated with the SCR technology has up to now prevented its widespread use, in particular in passenger cars. As an alternative to ammonia, there may also be used alcohols, hydrogen or hydrocarbons as reducing agent. These reducing agents however have considerably worse selectivities than ammonia for the nitrogen oxide reduction in the lean exhaust gas. Thus, a nitrogen oxide conversion of up to 30% is obtained in officially prescribed driving cycles using the alternative reducing agents, whereas conversions of 70% or more are possible with ammonia.
The selective catalytic reduction with ammonia thus provides very good results, but involves a considerable expenditure on equipment, which up to now has limited its widespread use in smaller engines.
It is therefore an object of the present invention to provide a process for the selective catalytic reduction with ammonia that is characterized by a simple production of the ammonia required for the reduction.
The above and other objects can be achieved by a process for the selective catalytic reduction of the nitrogen oxides contained in a lean exhaust gas from internal combustion engines with one or more cylinders, by reducing the nitrogen oxides by means of ammonia on a reduction catalyst. The process is characterized by the following process stages:
a) production of a rich gas stream with a normalized air/fuel-ratio of less than 1,
b) formation of ammonia in the rich gas stream by reaction of its components with one another,
c) combination of the lean exhaust gas with the rich gas stream, and
d) reduction of the nitrogen oxides contained in the lean exhaust gas on a reduction catalyst using the resultant ammonia as reducing agent.
The normalized air/fuel-ratio (xcex) describes the composition of the gas stream, and refers to the air/fuel ratio standardized to stoichiometric conditions. Stoichiometric conditions exist at a normalized air/fuel-ratio of 1. With a normalized air/fuel-ratio greater than 1 the gas contains more oxygen than is necessary for a complete combustion of the combustible constituents. Such a gas composition is termed lean. A rich gas composition exists when the oxygen content is less than is required for a complete combustion of all combustible constituents of the gas.
An essential feature of the process according to the invention is the production of the ammonia required for the catalytic reduction from a rich gas stream by the reaction of its components with one another. Such a gas stream may be produced for example by a burner that is operated with a sub-stoichiometric air/fuel mixture (xcex less than 1). The rich gas stream can also be obtained as part of the exhaust gas from the combustion engine if one cylinder of the engine is operated with a sub-stoichiometric air/fuel mixture. It is also possible to form the rich gas stream by injecting hydrocarbons into an air stream.
A rich exhaust gas contains for example, in addition to noncombusted hydrocarbons, also carbon monoxide, nitric oxide and water vapour. Ammonia can be formed from these last three substances according to the following reaction equation:
5CO+2NO+3H2Oxe2x88x92 greater than 5CO2+2NH3xe2x80x83xe2x80x83(1)
Nitric oxide is thus reduced by means of carbon monoxide to ammonia. The formation of ammonia is not restricted to a chemical reaction according to the above overall reaction equation. For example it is also possible to react hydrogen with nitrogen-containing gas components or with nitrogen to form ammonia.
This reduction may be carried out in various ways. It is possible to initiate the above reaction merely by thermal activation, in other words by heating the rich exhaust gas. Of course, the reverse reaction also increases with increasing temperature, and for this reason reaction pathways in which the exhaust gas does not have to be thermally heated are more favourable. An example of a convenient reaction pathway is to carry out the reaction on a suitable catalyst. As a rule the catalytic reaction requires lower temperatures, which means that the influence of the reverse reaction can be reduced.
It has now been found that ammonia can also be formed in a rich gas stream by passing the latter through an electrical gas discharge plasma. The formation of ammonia in an electrical gas discharge is thermodynamically favoured since the reaction proceeds at substantially lower temperatures than the catalytic reduction.